Method of manufacturing a composite, heat-insulating material of the type formed by stacking foils of oxidisable metal



United States Patent 3,284,256 METHQD 0F MANUFMCTURING A COMPOSITE, HEAT-INSULATENG MATERIAL OF THE TYPE FORMED BY STACKIING FUELS 0F UXIDTSAELE METAL Lucien Alfille, Orsay, and Raymond Darras, Versailles,

France, assignors to Commissariat a llEnergie Atomique, Paris, France No Drawing. Filed Oct. 3, 1961, Ser. No. 142,504 Claims priority, application France, Oct. 10, 1960, 840,707 6 Claims. (Cl. 156-11) The present invention relates to a method of manufacturing a composite heat-insulating material of the type formed by the stacking of thin metal foils having at least one of their surfaces covered by a heat insulating substance. It also relates to the product obtained by this method.

Heat-insulating materials, which may easily be deformed mechanically, are generally in the form of foils, strips, or ribbons within the structure of which there is a heat insulating substance generally in the form of refractory oxides, and a metal constituent giving these materials good mechanical properties.

Composite heat-insulating materials are, for example, formed by stacking a variable number of mixed layers, each layer being formed by a metal film having on at least one surface thereof, an insulating film, generally an oxide which is continuous and adheres to the underlying metal. Such materials may be manufactured, for instance, by the anodic oxidation of aluminum or nickel foils. However, the principal drawback of this method of manufacture is the fact that, when the heat-insulating material is Wound about an object to be covered, a discontinuity in the film of oxide adhering to the metal film is produced because of cracking. This drawback is difiicult to avoid since the manufacture of each mixed layer must be done before the object is covered.

Another method of making composite, heat-insulating materials consist in depositing on metal foils, a refractory heat insulating substance in the fibrous or powdery state made into a paste with a binding medium, and then in stacking the foils thus coated and destroying the binding medium and joining the refractory fibres to the metal by raising the temperature.

According to the present invention, the method of manufacturing a composite heat-insulating material of the aforementioned type consists in taking metal foils of the same type, but of diiferent thicknesses, forming a stack of alternately thick and thin metal foils about the object to be thermally insulated, and oxidising said stack at a high temperature so as to form a layer of refractory oxide between the thick foils by means of the total oxidation of the thin foils.

The composite, heat-insulating material obtained by this method, is essentially characterised in that the heat insulating layers covering the uncorroded metal portions are constituted by refractory oxides, the metal of which is of the same type as said metal portions. The oxides form a continuous and homogeneous adhesive layer on the surface of the metal portions.

The stacking of the different metal foils is always performed before actually heat-treating the foils, thus permitting the composite material to be directly adapted in shape to the objects to be heat-insulated This advantage is particularly important in the case of foil strips or ribbons surrounding the object to be insulated and formed by a number of mixed layers. Of course, the invention may also be applied in the case of stacks in the form of plates, which are in principle substantially plane.

Under given conditions of temperature and, particu- 3,284,256 Patented Nov. 8, 1966 larly, pressure means are provided for ensuring the coherence of the stacked arrangement before the oxidising treatment.

The winding of the strips or ribbons about an object to be insulated is preferably done without covering the dilferent windings by the same ribbon or strip. In order to obtain the correct and necessary stacked arrangement so as to have a high heat-insulating power, several windings are made which are superimposed and. are wound alternately in opposite directions. In this manner the metal foils which are conductive define isothermal surfaces, which is very favourable for the purpose of improving the heat insulation by avoiding any heterogeneous distribution of temperature at the same level.

The metal foils used to form the insulating material may be of any oxidisable material. The insulating material according to the invention may be used under radiation, for example, in nuclear reactors, and the metal foils may therefore be made of a material which is a bad neutron absorber, for example, magnesium, aluminum, or zirconium. In this case the refractory constituents, which are magnesia, alumina and zirconium, are also bad neutron absorbers.

External mechanical actions, such as crushing pressure, vibration, and fluid circulation, have only a limited effect as far as the destruction of the homogeneity of the heatinsulating properties of the material obtained by the method according to the present invention is concerned. In fact, this elfect is combated by the metal foils which prevent any sinking or breakage in the insulatory homogeneity of the material.

On the other hand it should be stated that this material has an excellent geometrical behaviour under load and temperature, as a result of its mechanical qualities, so that the problem of securing it for example, in nuclear reactors, to pressure tubes thermally insulated from the fuel element guide tube, which is at a high temperature, may easily be solved.

The heat-insulating material obtained by the method according to the present invention has the following principal advantages:

Ease of use.

Definite shaping in the operative conditions.

The coefiicient of heat conductivity, in the c.g.s. system, is lower than 3l0 cal./cm./a./ C

Very high resistance to thermal shock.

Very high resistance to sharp variations in pressure.

Very good mechanical behaviour when subject to vibration.

No deterioration under radiation.

Excellent distribution of materials, such .as magnesia and alumina, between the thick foils.

Very strong apparent density of materials, such as, alumina and magnesia, which, without harming the insulating power, aids mechanical behaviour.

Definite passivation in relation to the chemical and/or physical agents existing in nuclear reactors.

The following are examples of the method of manufacturing a composite, heat-insulating material according to the present invention:

EXAMPLE I In this example magnesium foils having thicknesses of 0.15 mm. .and 0.08 mm. are used. First of all a thick foil is wound around an object to be insulated, then a thin foil, and so on until the desired stack is obtained. The last foil wound has to be a thick foil. Over the outside of this stack is wound a ribbon of zirconium which is welded at the ends, thus forming a reinforcement.

The stacked arrangement thus obtained is then subjected to a temperature of 500 C. in an atmosphere of carbon dioxide at a pressure of 60 kg/em. (with natural convection and forced circulation).

The thin foils, which have a thickness of 0.03 mm., are completely oxidised, whilst the thick foils are only partially oxidised on their two surfaces. Thus between two thick foils a homogeneous layer of magnesium oxide is obtained which i's'continuous, has a thickness in the region of 0.09 mm. and constitutes an excellent refractory heat insulator.

EXAMPLE II The conditions are the same as in the foregoing example, but the magnesium is replaced by aluminum; the insulating constituent of the stacked arrangement is then alumina.

EXAMPLE III As in the case of the first example, the foils are made of magnesium, the insulator being magnesia; the thickest foils have a thickness of 0.15 mm., and the thinnest foils have a thickness of 7 mm. The oxidization of these foils is carried out at a temperature of 450 C. under a pressure of 25 kg./cm This last method allows the effects of corrosion on thick foils to be avoided in the case of use of the heat-insulating material in a nuclear reactor where the moderator is carbon dioxide.

In all cases, the heat treatment always takes place after performing the stacking and the stack always has its final form when the heat treatment is carried out.

We claim:

1. In a method of manufacturing a composite, heatinsulating material comprising layers of foils of an oxidisable metal in which each layer has at least one surface covered by a heat insulating substance, the steps of forming a stack of alternately thick and thin foils of the same metal having low neutron absorption around an object to be thermally insulated, said metal being selected from the group consisting of magnesium and aluminum and then completely oxidizing the thin metal foils at an elevated temperature between 350 C. and 525 C. in an atmosphere of carbon dioxide at a pressure higher than atmospheric pressure between 15 kg./cm. and kg./cm. to form a layer of refractory oxide between the thick foils.

2. A method as described in claim 1 wherein the thickness of the thick foils is between 0.1 mm. and 0.5 mm. and the thickness of the thin foils is between 0.01 and 0.1 mm.

3. A method as described in claim 1 wherein the foils are of magnesium and the thickness of the thick foils is on the order of 0.15 mm.; the thickness of the thin foils is on the order of 0.03 mm.; and oxidation is carried out at 450 C. and at a pressure of 25 kg./cm.

4. A method as described in claim 1 in which the stack of metal foils is externally reinforced.

5. A method as described in claim 4 in which the metal foils are reinforced by thin ribbons of zirconium.

6. A method as described in claim 1 in which the stack of foils is formed by winding the foils about the object alternately in opposite directions.

References Cited by the Examiner UNITED STATES PATENTS 2,088,949 8/1937 Fekete 29-195 2,506,364 5/1950 Jarnie et a1 148-20.3 2,784,123 3/1957 Rappapoet 1486.3 3,050,843 8/ 1962 Margolis et al 29-470 JACOB H. STEINBERG, Primary Examiner.

EARL M. BERGERT, Examiner.

R. J. ROCHE, Assistant Examiner. 

1. IN A METHOD OF MANUFACTURING A COMPOSITE, HEATINSULATNG MATERIAL COMPRISING LAYERS OF FOILS OF AN OXIDISABLE METAL IN WHICH EACH LAYER HAS AT LEAST ONE SURFACE COVERED BY A HEAT INSULATING SUBSTANCE, THE STEPS OF FORMING A STACK OF ALTERNATELY THICK AND THIN FOILS OF THE SAME METAL HAVING LOW NEUTRON ABSORPTION AROUND AN OBJECT TO BE THERMALLY INSULATED, SAID METAL BEING SELECTED FROM THE GROUP CONSISTING OF MAGNESIUM AND ALUMINUM AND THEN COMPLETELY OXIDIZING THE THIN METAL FOILS AT AN ELEVATED TEMPERATURE BETWEEN 350* C. AND 525* C. IN AN ATMOSPHERE OF CARBON DIOXIDE AT A PRESSURE HIGHER THAN ATMOSPHERIC PRESSURE BETWEEN 15 KG./CM.2 AND 60 KG./CM.2 TO FORM A LAYER OF REFRACTORY OXIDE BETWEEN THE THICK FOILS. 